1. Field of the Invention
The present invention relates to an optical pickup having a semiconductor laser or the like as a light source in which a light flux from the light source is irradiated onto an optical information recording medium, therby performing either one of or both of recording and reproduction of information.
2. Related Background Art
According to an optical pickup for optically performing either one of or both of recording and reproduction, a light beam from a light source is converged to a microspot beam by an optical system and irradiated onto an information recording medium, and the light beam is intensity modulated by a recording signal, thereby changing an optical characteristic of the recording medium and recording information. On the other hand, the light beam of a weaker intensity than that upon recording is also converged to a microspot beam similar to the case of recording and irradiated onto the medium, and an optical change of the medium is detected, thereby reproducing information. An optical recording and reproducing apparatus using such an optical pickup has various features such that density of information can be raised and the like. However, when information is recorded and reproduced, a light beam is always converged into a microspot beam and irradiated onto an information recording medium, so that it is necessary to perform a focus control for keeping the distance between the information recording medium and the optical system constant and a tracking control for tracing the track on which information has been recorded. In general, in an optical recording and reproducing apparatus, in order to obtain an informtion signal when reproducing information by using a reflecting type disk, and in order to derive a control signal when performing the foregoing focus control and tracking control, the reflected light needs to be taken out of the information recording medium.
In general, a beam splitter is used to take out reflected light from the information recording medium. The light beam is directed onto the optical path from the light source to the recording medium by the beam splitter, the light of the light source is efficiently transferred to the recording medium and the light reflected by the recording medium is efficiently separated from the optical path, thereby detecting the information signal and control signal.
However, the conventional optical recording and reproducing apparatus using the beam splitter has a drawback such that a stray light which fairly decreases the S/N ratios of the information signal and control signal, particularly, a ghost light is generated by the beam splitter.
To solve this drawback, the applicant of this invention has proposed an optical pickup (U.S. Pat. No. 4,907,858: same assignee). According to this optical pickup, among six planes constituting the beam splitter, the plane opposite to the plane which faces the direction of a photodetector is set to the plane which does not reflect a light beam from a light source to the direction of the photodetector, namely, to the plane which is inclined for the direction of the photodetector, thereby preventing the ghost light. However, according to the beam splitter used in this optical pickup, in the case of using a plurality of photodetectors which are almost rectilinearly arranged as in a three-beam system, the light beam is influenced by a distribution shape of the reflected beam light fluxes, so that there is a problem such that the generation of the ghost light cannot always be prevented and the S/N ratio in detection of the signal deteriorates. This problem will now be described hereinbelow with respect to an optical recording and reproducing apparatus using the astigmatism system for the focus control and using the 3-beam system for the tracking control with reference to FIGS. 1 to 3(a) and 3(b). In FIG. 1, the light fluxes emitted from a semiconductor laser 1 are converted into parallel light fluxes by a collimator lens 2 and divided into three beams by a diffraction grating 3. The divided light beams are transmitted through a beam splitter 4 and converged onto a disk 6 by an objective lens 5, so that three beam spots are formed. The light beam reflected by the disk 6 again passes through the objective lens 5 and is reflected by the beam splitter 4 and separated from the incident beam. The reflected beam is converged by a condenser lens 7 and a cylindrical lens 8 and enters photodetectors 9.sub.1, 9.sub.2, and 9.sub.3. The photodetectors 9.sub.1, 9.sub.2, and 9.sub.3 are arranged so as to receive the lights from the three beam spots formed on the disk 6, respectively.
As shown in FIG. 2, the photo sensing surface of the photodetector 9.sub.2 is divided into four areas. By obtaining the difference between the sum of the detection signals of a pair of photo sensing surfaces arranged on a diagonal line and the sum of the detection signals of another pair, the astigmatism caused by the cylindrical lens 8 is detected. Thus, a focus control signal is derived by the well-known principle of the astigmatism system. On the other hand, in the case of reproducing information, a reproduction signal is derived from the photodetector 9.sub.2. Further, by calculating the difference between two detection signals of the photodetectors 9.sub.1 and 9.sub.3, a tracking control signal is obtained by the well-known principle of the 3-beam system.
FIGS. 3(a) and 3(b) show the states of the reflected beams on the photodetectors 9.sub.1, 9.sub.2, and 9.sub.3 in the case when a plane 4.sub.1 opposite to the plane which faces the direction of the photodetectors among six planes constituting the beam splitter 4 is inclined so as to avoid the ghost lights in the direction which is almost parallel with the arrangement direction of the photodetectors 9.sub.1, 9.sub.2, and 9.sub.3 in order to prevent the ghost lights from entering the photodetectors 9.sub.1, 9.sub.2, and 9.sub.3.
FIG. 3(a) is the diagram showing the case when the plane 4.sub.1 of the beam splitter is slightly inclined and the ghost lights are avoided at the intervals among the photodetectors 9.sub.1, 9.sub.2, and 9.sub.3. In this case, there is a large possibility of the occurrence of leakage of the ghost lights to the photodetectors 9.sub.1, 9.sub.2, and 9.sub.3 and the S/N ratios of the signals deteriorate. On the other hand, as shown in FIG. 3(b), there is also considered a method whereby the plane 4.sub.1 of the beam splitter is largely inclined to thereby largely deviate the ghost lights from the photodetectors 9.sub.1, 9.sub.2, and 9.sub.3. However, when the angle of inclination increases, there is a possibility that the light fluxes are further reflected by an optical column wall portion and the like in the way of the optical path and are irradiated onto the photodetectors 9.sub.1, 9.sub.2, and 9.sub.3, so that they become a cause of the occurrence of the ghost lights. To prevent this problem, a method whereby a large enough space is assured on the optical path is considered. However, according to this method, the size and weight of the optical pickup increase.
On the other hand, the foregoing problems also obviously occur in the case when two photodetectors are provided as in the well-known wedge prism system (focus control).